1. Field of the Invention
This invention relates generally to the control of the supply pressure of a hydraulic actuator, and more particularly to a control for the supply pressure of a hydraulic actuator of automatic transmission for motor vehicles.
2. Description of the Prior Art
According to the prior art, standard automatic transmissions, equipped with a coupling or torque converter, are so designed that, when the vehicle is in forward (or reverse) gear and the engine is running, the transmission of the engine torque to the wheels is not interrupted, even when the vehicle is stopped. This results in a certain number of consequences, some of which can be classed as advantages of this type of transmission, but others can be considered as drawbacks; moreover, some of these advantages can become disadvantages because of driving conditions and vice versa.
This lack of interruption of the torque between the engine and wheel is reflected, when the vehicle is stopped with the engine idling, by a drag torque, which has a magnitude which depends on the characteristics of engine and the coupling element (torque converter or coupling), which gives the vehicle a spontaneous tendency to creep. This tendency to creep can be considered as favorable to the extent that:
it allows smooth, exact movements of the vehicle at very low speed (parking maneuvers, for example);
it prevents the vehicle from rolling back when starting on a moderate grade.
On the other hand, it constitutes an undeniable drawback to the extent that:
it makes it necessary to keep the foot constantly on the brake to keep the vehicle still after stopping on a level road;
it makes it necessary to set the idle higher than would be necessary for a true "neutral," which, in city traffic, leads to considerable consumption at each stop of the vehicle with either a gasoline or diesel engine;
the lack of interruption of the torque between the engine and wheels puts the entire corresponding kinematic linkage under mechanical stress, subjects it to vibrational stressing coming from the engine; this results in shaking that is often unacceptable in the passenger compartment, especially for vehicles equipped with a diesel engine.
In view of these phenomena, the need has been felt for a device that can be engaged at will by the driver, a device that automatically "disengages" the transmission each time the vehicle is stopped and "reengages" it with gradually at each restarting. Simply designed devices are known which correctly assure the function of disengaging the transmission, upon stopping, by cutting off the supply of the oil to the cylinder which assures locking of the input friction element of the transmission; but these devices do not take into account various speed parameters of the automobile (engine, turbine of the coupling element, vehicle). This results in a free wheeling effect that is more or less undesirable and in any case uncontrollable, and a non-gradual return to torque. Actually, at the moment of reengagement, when the driver presses on the accelerator, the cylinder is resupplied with pressure without particular care and with no relation to the change of the engine speed so that coupling may be abrupt, premature or late (causing racing of the engine).